1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a facsimile, a printer, and a multifunction peripheral (MFP) and a transfer device that uses an electrostatic image transfer process, and more particularly, relates to a technology for enhancing a neutralizing mechanism for members arranged in the vicinity of the transfer device used in the image forming apparatus.
2. Description of the Related Art
Generally, in an image forming apparatus, a toner image is formed on an outer peripheral surface of image carrying members such as a photosensitive drum, a photosensitive belt, and an intermediate transfer belt by using an exposure device and a developing device based on an electrophotographic method. Furthermore, an electric field having a reverse polarity with respect to a charging polarity of the toner image is formed in the image forming apparatus by applying a transfer bias (transfer voltage). The image forming apparatus includes a transfer device that generates an electrostatic attractive force by forming the electric fields as described above and that transfers the image from the image carrying members onto sheet materials such as copy sheets, thick sheets, and resin sheets including OHP transparencies (an electrostatic repulsive force can also be used by using an electric field having the same polarity with the toner image).
Thus, the transfer device, which uses the electrostatic image transfer process, uses a transfer method that is broadly classified into a direct transfer method and an indirect transfer method. In the direct transfer method, the toner image is formed on multi-color or mono-color latent image carrying members such as the photosensitive drum and the photosensitive belt by using a developing unit and the toner image is directly transferred onto the sheet material from the latent image carrying members that are the image carrying members for carrying the toner image. In the indirect transfer method, the toner image is once transferred onto an intermediate transfer body such as the intermediate transfer belt from the multi-color latent image carrying member by a primary transfer unit and subsequently, the toner image is collectively transferred onto the sheet material by a secondary transfer unit.
The transfer device also uses a contact method and a non-contact method. In the contact method, the transfer bias is applied by causing transfer bias applying units such as transfer rollers (a transfer roller of the direct transfer method and a primary transfer roller and a secondary transfer roller of the indirect transfer method) to directly contact transfer electric field forming target objects (belt members such as a conveying belt of the direct transfer method and the intermediate transfer belt of the indirect transfer method, and the sheet materials). In the non-contact method, the transfer bias is applied without causing the transfer bias applying unit such as a corona discharger to contact the transfer electric field forming target object.
However, the transfer device using the direct transfer method and the transfer device using the indirect transfer method commonly use the electrostatic image transfer process, in other words, form the transfer electric field by applying the transfer bias by using the transfer bias applying unit. Thus, the transfer device needs to be formed such that members, which contact at the time of applying the transfer bias, the transfer electric field forming target objects such as the intermediate transfer belt and the sheet material or which are within the range of fixed distance from the transfer electric field forming target objects such as the intermediate transfer belt and the sheet material, include an electric resistance (volume resistance) that is greater than or equal to a fixed value such that the transfer bias is not leaked.
Thus, if the transfer device includes the members including the resistance greater than or equal to a fixed value, naturally generated static electricity is less likely to flow and escape to another members. Due to this, the sheet material is repeatedly conveyed and due to friction (including contact, separation, and collision, hereinafter the same) with the sheet material or air, the member is positively or negatively charged. The generated static electricity is accumulated on the members, thus resulting in occurrence of bias in electric potential. If the static electricity is accumulated on the members that are within the fixed range (generally, a shortest distance is within five millimeters (mm)) from the sheet material, thus resulting in high potential static electricity, the sheet material is partially charged due to contact or electric discharge between the members and the sheet material before transferring the toner image onto the sheet material. Furthermore, when the sheet material carrying the toner image passes through the vicinity of the members after the toner image is transferred onto the sheet material, an unfixed toner image spoils due to the high potential static electricity accumulated on the members, thus deteriorating image quality such as toner scattering and raggedness of the image.
In other words, because the members are within the fixed range from the transfer electric field forming target object, the members need to be formed by including an electric resistance that is greater than or equal to a predetermined value to ensure that transfer current (or voltage) is not leaked. The members (hereinafter, called conveying path proximal members), which are arranged opposite a conveying path of the sheet material and are likely to charge due to friction with the sheet material or air, need to be neutralized by arranging a neutralizing mechanism. In the neutralizing mechanism, the members are ground connected by using a conducting material and earthed. However, for neutralizing the conveying path proximal member, a minimum resistance from the conveying path proximal member until ground connection of the conveying path proximal member is reduced such that the charged static electricity can flow easily. However, the conveying path proximal member is formed of a material of a high resistance to ensure that the transfer bias is not leaked. In other words, the conveying path proximal member necessitates the high (electric) resistance from the viewpoint that the transfer bias is not leaked and also necessitates a low resistance from the viewpoint that failure occurring due to charging can be prevented. In other words, it is necessary to simultaneously overcome two drawbacks that are inversely related to each other.
Whether the conveying path proximal member is to be charged positively or negatively is determined by a sequence of triboelectric series between a material of the sheet material and a material of the conveying path proximal member, in other words, based on a relative ease of emitting electrons. For example, when the sheet material is formed of a paper and the conveying path proximal member is formed of polyethylene resin, as compared to polyethylene resin, the triboelectric series for the paper is on a plus side, thus negatively charging the conveying path proximal member.
As an example of the existing image forming apparatus that includes the neutralizing mechanism, an image forming apparatus that is disclosed in Japanese Patent Application Laid-open No. H9-40225 is explained. The image forming apparatus transfers a toner image on an image carrying member onto a transfer material by using a transfer roller. In the image forming apparatus in which the toner image is separated from the transfer material that is neutralized by a neutralizing needle unit, if a separation detecting sensor detects that the transfer material is incorrectly separated from the image carrying member, a controller changes over a switch, thus changing over bias voltage from high-voltage power supplies to neutralizing needles. Based on environmental conditions, quality of a transfer material, usage conditions of neutralizing needles, optimal neutralizing conditions are set and to deal with changes in charge-retaining force due to dampness of the transfer material, changes in charge-retaining force due to change in the resistance according to the material of the image carrying member, adhesion of toner on points of the neutralizing needles, and degradation of neutralizing properties due to electric discharge, the transfer material is separated from the image carrying member that is neutralized by optimal conditions.
However, in this image forming apparatus, for separating the transfer material, which is the sheet material, from a belt, the transfer material is neutralized by the neutralizing needles. By effectively changing over the high-voltage power supplies, potential of the transfer material can be controlled. However, occurrence of image degradation by charging the conveying path proximal member is not taken into consideration in the image forming apparatus.
As another example of the existing image forming apparatus, an image forming apparatus that is disclosed in Japanese Patent Application Laid-open No. H11-7200 is explained. The image forming apparatus includes a primary charger (charging unit), an image exposing unit (image data writing unit), a developer (developing unit), a transfer roller (transferring unit) that transfers the toner image formed by the developer onto a transfer material, neutralizing needles (separating unit) that facilitate separation of the transfer material after transfer from a photosensitive drum. In the image forming apparatus, a contact transfer method is adopted in the transfer roller and a transfer bias power supply (transfer bias supplying unit) that supplies the transfer bias to the transfer roller and a separation bias power supply (separation bias supplying unit) that supplies the separation bias to the neutralizing needles are arranged. The neutralizing needles are ground connected via a variable resistance body or a conducting switch and according to a transfer bias supply starting time of the transfer bias power supply (transfer bias supplying unit), the resistance of the variable resistance body is set smaller than the resistance after the transfer bias supply starting time or according to the transfer bias supply starting time, a conducting switch is conducted and conduction is released at a predetermined time.
However, in this image forming apparatus, by arranging the variable resistance body, a plurality of resistors and controlling units thereof are required for changing over the resistance of the variable resistance body. Therefore, a structure of the image forming apparatus becomes complex, thus increasing a cost. Furthermore, the entire image forming apparatus cannot be made compact.